A field experiment was carried out at the Teaching and Research farm of the University of Agriculture, Abeokuta, located between longitude 7°15’N and latitude 3°25’E, a derived savannah in south western Nigeria. The study was carried out between April to September 2009 and March to August 2010 to investigate the effect of varying populations of two local cowpea varieties of contrasting growth habits on green manure production, using maize as test crop. Three population densities each of cowpea varieties Oloyin and Drum, and the control plots were arranged in a Randomized Complete Block Design and replicated three times. Six weeks after planting, the green manure was uprooted and incorporated in situ. The incorporated green manure was left for a week after which maize variety- SUWAN-IY was planted on all plots. All populations of cowpea variety Oloyin gave significantly higher biomass than all the populations of Drum in 2009; the highest significant fresh biomass was produced at 111,111 plants/ha. In 2010, Drum at 160,000 plants/ha and Oloyin at 111,111 plants/ha produced similar plant biomass (p>0.05) and were significantly higher than all other treatments. However, cowpea variety Drum at 80,000 plants/ha and Oloyin at 55,555 appeared to be more economical relative to other treatments in terms of seed requirements for optimum biomass production and maize grain yield. In 2009 maize grain yield was not significantly different between treatments. In 2010 however, both maize grain yield and cob girth were significantly increased (p<0.05) on green manure plots. Maize grain yield increased by 37-98% and 89-147% on green manure plots in 2009 and 2010, respectively relative to the control plots. In maize production systems in the derived savannah of Nigeria, 55,555 plants/ha of Oloyin and 80,000 plants/ha of Drum could be recommended for green manuring. This will be of immense benefit to the resource- poor farmers in this ecological zone.